Technology update

May 27, 2011

'Electric glue' helps stick polymers to electrodes

"Electric glue" can be used to reversibly stick polymers to the surface of an electrode to create nanoscale devices that can be switched on and off using a current. Researchers recently showed that DNA (a biopolymer) can be reversibly bonded to a gold electrode by varying the electric potential of the electrode surface. Now, a team at Ludwig-Maximilians University in Germany has gone a step further by looking at non-covalent interactions between the polymer and the electrode – a study that could be important for applications such as biosensors.

Electric glue can be activated or inactivated using applied voltage and bonding may even be designed to work in the opposite way. For example, an applied voltage could actually be used to weaken the interactions between surfaces so that they separate.

Hermann Gaub and colleagues employed an atomic force microscope (AFM) with a working gold electrode. The researchers covalently attached different polymers to the AFM tip and brought it into contact with the gold electrode surface. They then moved the polymers away from the electrode and measured the resultant force while changing the potential of the electrode in steps of 10 mV. The potential of the electrode regulates oxidation and reduction reactions at the surface of the electrode.

Gaub and co-workers studied three different types of polymer: the neutral polyethylene glycol (PEG), 2,2-ionene (which has a positively charged backbone) and a negatively charged biopolymer – double-stranded DNA where three of the four bases contain primary amines.

Roburogram
The researchers plotted the data obtained in a "roburogram" ("robur" is the Latin for force), where rupture force was plotted against potential. Such a plot is similar to a cyclic voltammogram except that the y-axis on the graph is the rupture force rather than current.

They found that there was little interaction between the polymer and the electrode near high applied potentials of 1 V (because of electrode oxidation) but that sticking increased as the potential was reduced. Indeed, interaction reactions were found to begin at potentials around 0.3 V for the PEG and 2,2-ionene. However, in the case of DNA, the potential had to be lowered more to allow for a build-up of charged ions on the surface of the electrode to mediate the interaction between the negatively charged backbone of the DNA and the negative applied potential of the electrode, explains lead author Ann Fornof.

"Glues adhere to solid materials via a multitude of fundamental physical or chemical interactions," she told nanotechweb.org. "In our work, we describe how we can control the interaction of polymers with surfaces under applied potential based on a number of factors including Coulomb forces and oxidation/reduction of the surface. Since these interactions can be controlled by an applied electric potential to the surface, it is possible to externally control the adhesion of polymers to an electrode surface."